Mastering Concepts
33.1
1. List the cell types that are important in the body’s defenses, along with some of their
functions.
Neutrophils, eosinophils, and monocytes act as phagocytes; basophils and mast cells
release chemicals to trigger inflammation and allergies; lymphocytes (T cells and B cells)
coordinate defenses against specific pathogens. Natural killer cells attack cancerous and
virus-infected cells.
2. What is the relationship between lymph and blood?
Lymph is blood plasma that has leaked out of capillaries and is traveling in the lymphatic
system, eventually to be cleaned, filtered, and returned to the bloodstream.
3. What are the main lymphoid organs, and what are their functions?
The main lymphoid organs are the thymus, spleen, lymph nodes, adenoids, and tonsils.
All of these lymphoid organs have a similar function: they are sites with high
concentrations of lymphocytes and macrophages. In lymphoid organs, lymph is cleaned
and filtered, and bacteria and tissue debris are removed.
4. What are the two main subdivisions of the immune system?
The immune system is divided into innate defenses and adaptive immunity.
33.2
1. List five categories of innate defenses.
Barriers, white blood cells, inflammation, the complement system, and fever are the
innate defenses.
2. What are some barriers to infection in the human body?
Barriers to infection in the human body include the skin, mucous membranes in the nose
and throat, earwax, tears, cilia in the respiratory system, acid secretions of the stomach,
and the normal bacteria in the gut and elsewhere.
3. How do white blood cells and macrophages destroy invaders?
White blood cells destroy invaders by phagocytosis, triggering inflammation, or secreting
chemicals that destroy pathogens.
4. Describe the events of inflammation.
Basophils and mast cells secrete histamine, causing blood vessel dilation and increased
permeability. The phagocytic white blood cells leave the blood and enter the tissue to
destroy bacteria and damaged cells.
5. What are some examples of antimicrobial biochemicals?
Examples of antimicrobial biochemicals include complement proteins that destroy
pathogens, attract phagocytes, or cause mast cells to release histamine.
6. How is fever protective?
Fever creates an environment that is hostile to bacteria and viruses. Not only is the
warmer temperature lethal to some bacteria and viruses, it also reduces the iron level in
the blood, depriving bacteria and fungi of iron and reducing their replication. Fever also
increases the activity level of phagocytes.
33.3
1. What is the relationship between antigens and antibodies?
Antigens are molecules that trigger the production of antibodies.
2. What is the role of macrophages in adaptive immunity?
In adaptive immunity, the macrophages engulf invading cells and then act as “teachers”
that display foreign antigens on their surfaces. Helper T cells recognize these antigens
and bind to the macrophage. This initiates other phases of the cellular response and
humoral response.
3. What are the two subdivisions of adaptive immunity, and which cell types participate
in each?
The two subdivisions of adaptive immunity are the cellular response and the humoral
response. Helper T cells, memory T cells, and cytotoxic T cells participate in the cellular
response. Memory B cells and plasma cells participate in the humoral response.
4. What do cytotoxic T cells do?
Cytotoxic T cells engulf and destroy cancerous cells or cells that are infected with
viruses.
5. Describe the structure and function of an antibody.
An antibody is a Y-shaped protein that matches a specific antigen. Upon encountering an
invader with a matching antigen, the antibody binds to the antigen. This may make the
antigen more noticeable to macrophages, may inactivate a microbe, or may neutralize its
toxins. Viruses that are coated with antibodies may not be able to connect with their
target cells. Antibodies also trigger the production of complement proteins that destroy
microbes.
6. How can a relatively small number of genes encode billions of antibodies and antigen
receptors?
Developing lymphocytes can shuffle segments of genes to produce a tremendous number
of new combinations of antibodies and antigen receptors.
7. What happens after a B cell is stimulated?
A stimulated B cell can proliferate, producing memory cells, or it can produce plasma
cells that make antibodies.
8. What happens if an immune reaction persists after a pathogen is eliminated?
An immune system response that persists after a pathogen is eliminated may attack and
damage the body’s own tissues.
9. Explain the difference between the primary and secondary immune response.
The primary immune response is triggered the first time the body encounters a pathogen;
a secondary immune response is triggered on subsequent exposure. A primary immune
response may take days or weeks to respond to the infection. The secondary immune
response is much quicker and stronger than the primary response.
33.4
1. What is a vaccine?
A vaccine is a mixture containing antigens that stimulate the primary immune response
without actually causing disease.
2. List the main types of vaccine formulations.
Vaccines can be composed of live but weakened pathogens, inactivated pathogens or
toxins, subunits of pathogens, or recombinant component vaccines.
3. Why haven’t scientists been able to develop vaccines against HIV and the common
cold?
This viruses mutate too frequently for a vaccine to be effective.
33.5
1. What events might lead to autoimmunity?
One event that might lead to autoimmunity includes the survival of lymphocytes that
correspond to the body’s molecules. These ordinarily are destroyed early in development,
but if they survive, they can cause autoimmunity.
2. Which immune system cells does HIV attack, and what is the consequence?
HIV attacks helper T cells, causing the body’s immune response to fail. Once too many T
cells are lost, the person becomes susceptible to opportunistic illnesses and infections.
3. How does SCID differ from HIV infection?
SCID is different from HIV infection in that SCID is not caused by a virus; instead, SCID
children have a genetic lack of immune response. Their T cells and B cells do not
function and they are prone to opportunistic infections.
4. Which cells and biochemicals participate in an allergic reaction?
The cells that participate in an allergic reaction include B cells, plasma cells that secrete
antibodies, and mast cells. Biochemicals include histamine released from mast cells.
5. How is the Rh-factor important in determining whether a pregnant woman’s immune
system attacks her fetus?
Fetal cells can enter a woman’s bloodstream and stimulate the production of antibodies
against the Rh antigen. An Rh-negative mother’s immune system will form antibodies
against the red blood cells of an Rh+ fetus. The first pregnancy will be fine, but the
second pregnancy of an Rh+ fetus in an Rh- mother will result in the mother’s immune
system destroying the red blood cells of her fetus.
33.6
1. In this study, how did researchers document the inverse relationship between worm
infection and allergies?
The children were exposed to different allergens, including dust mites, on a small portion
of their arms. The presence of welts indicated a positive allergic reaction. The students
were also tested for the presence of parasitic worms. The researchers then graphed the
incidence of allergies among children with and without worms.
2. Describe the role of interleukin-10 in explaining the worm–allergy relationship.
IL-10 acts as a brake on the immune system, and it is released in response to the presence
of parasitic worms. Therefore, children with worms produce IL-10 and are less likely to
suffer from allergies.
Write It Out
1. List and describe the components of the lymphatic system.
The components of the lymphatic system are lymph (the colorless fluid that carries white
blood cells and that the lymphatic system transports, cleanses, and returns to the
bloodstream); lymphoid organs such as the thymus, spleen, and lymph nodes (collections
of lymphocytes embedded in loose connective tissue); and lymph vessels (tubes that
absorb and carry lymph).
2. Explain the observation that lymphoid tissues are scattered in the skin, lungs, stomach,
and intestines.
The skin, lungs, stomach, and intestines are all in contact with the outside world, so they
are places where pathogens are likely to enter the body.
3. How does the immune system interact with the circulatory system?
Phagocytes, antibodies, and other immune system cells and substances originate in the
immune system but travel in the bloodstream. In addition, lymph originates as blood
plasma that leaks out of blood vessels. Once cleansed, lymph returns to the blood.
4. List five types of innate defenses.
Innate defenses are always present and ready to function. They include physical barriers
such as intact skin, mucus, earwax, and tears; white blood cells; inflammation;
antimicrobial biochemicals; and fever.
5. Dead phagocytes are one component of pus. Why is pus a sure sign of infection?
Phagocytes fight infection by engulfing and destroying bacteria and damaged tissues. Pus
is a fluid containing these white blood cells and other debris.
6. How can inflammation be both helpful and harmful?
Inflammation can be helpful because it recruits immune components, helps clear debris,
and creates an environment hostile to microorganisms around the site of injury or
infection. It can be harmful as it can cause the site to become swollen and painful. Also,
joints and other body parts can become inflamed even if they are uninfected, causing
great discomfort.
7. If you take an anti-inflammatory drug after spraining your ankle, what symptoms
should be reduced?
The drug will keep the vessels from dilating and becoming more permeable; redness,
swelling, and heat should therefore be reduced. The drug should also relieve pain
because the lack of swelling in the tissues will keep pressure off surrounding nerves.
8. State the functions of antibodies, cytokines, and complement proteins.
Antibodies are Y-shaped proteins that recognize and bind to specific antigens; cytokines
are messenger chemicals that alter the activity of immune cells in many ways;
complement proteins help destroy pathogens in the body.
9. What do a plasma cell and a memory cell descended from the same B cell have in
common, and how do they differ?
Both plasma and memory cells are formed after activation of B cells; plasma cells secrete
huge number of antibodies, whereas memory cells “remember” antigens the immune
system has already encountered.
10. In your own words, write a paragraph describing the events of adaptive immunity,
beginning with a macrophage engulfing a bacterial cell and ending with the production of
memory cells.
When a macrophage engulfs a bacterial cell, it displays the bacterial antigens on its
surface. The antigen-presenting macrophage enters a lymph node, where the
combination of self protein plus bacterial antigen attracts helper T cells. The helper T
cells secrete chemicals that both B cells and T cells with receptors for the bacterial
antigen. The activated B cells divide and differentiate into plasma cells that produce
antibodies; some B cells also differentiate into memory cells. Likewise, activated T cells
differentiate into memory cells and cytotoxic T cells that attack the bacterium.
11. How do the innate defenses and adaptive immunity cooperate to eradicate an
infection?
Innate defenses include nonspecific barriers, inflammation, chemical defenses, and fever.
Phagocytosis is part of the body’s innate defense, but when macrophages display foreign
antigens on their surfaces, they trigger the adaptive immune response. In adaptive
immunity, immune cells produce antibodies and other defenses against specific antigens.
12. Which do you think would be more dangerous, a deficiency of T cells or a deficiency
of B cells? Explain your reasoning.
Both would be dangerous, but helper T cells coordinate the immune response (including
triggering the activity of B cells), so a deficiency of T cells would be more dangerous.
13. One benefit of sexual reproduction is a genetically variable population. According to
the red queen hypothesis, genetic variability helps a population stay “one step ahead” of
pathogen populations. Describe how genetic variability can enhance immunity.
Ultimately, the immune response has a genetic basis. The more genetic variability in a
population, the greater the potential to develop an immune response against a variety of
pathogens. In addition, many pathogens attack only cells with specific receptor
molecules. Genetic variability in the host population means more variants of those
receptors, which makes it more difficult for the pathogen to constantly stay compatible.
14. What is a vaccine, and how is a vaccine different from an antibiotic?
A vaccine is a substance that triggers a primary immune response, whereas an antibiotic
is a drug that kills bacteria. A vaccine is different from an antibiotic because it does not
destroy the pathogen. In addition, a vaccine triggers the production of memory cells that
can quickly respond to a future infection.
15. Explain why it might be dangerous for a person with a weakened immune system to
receive a vaccine consisting of a live, weakened pathogen.
A live, weakened pathogen triggers a mild version of the illness. Most individuals would
be able to handle such an infection, but an individual with a weakened immune system
could become very ill.
16. Influenza viruses mutate rapidly, whereas the chickenpox virus does not. Why are
people encouraged to receive vaccinations against influenza every year, whereas
immunity to chickenpox lasts for decades?
Vaccines work by “teaching” the recipient’s immune system to recognize a diseasecausing organism without actually causing the illness. Viruses have surface molecules
that the immune system recognizes. If the virus frequently mutates, frequent vaccines are
required to “teach” the immune system about the new variants. The memory of the slowto-mutate chickenpox virus lasts much longer.
17. If worm infections suppress the immune system as suggested in section 33.6, do you
think vaccines should be more or less effective in children infected with worms? Explain
your answer.
Vaccines should be less effective in children with worms because vaccines trigger an
active immune response, which would be suppressed.
18. What is an opportunistic infection? Explain the statement that opportunistic
infections, not HIV alone, cause death in an end-stage AIDS patient.
Opportunistic infections result from pathogens that the immune system can normally
defeat. These pathogens can, however, cause infection in people with weakened immune
systems. In end-stage AIDS patients, HIV has destroyed most helper T cells. The patient
dies from opportunistic infections their immune system can no longer defeat.
19. How does each of the following illnesses affect immunity?
a. SCID b. AIDS c. Hay fever
(a) SCID is an inherited condition in which neither the T nor B cells function. (b) AIDS
is caused by the HIV virus, which attacks and destroys helper T cells in the body. (c) Hay
fever is an allergic condition in which the immune system responds dramatically to a
harmless antigen in pollen.
20. What part do antibodies play in allergic reactions and in autoimmune disorders?
In autoimmune disorders, antibodies attack the body’s own cells. In allergic reactions,
antibodies bind to mast cells, causing them to release histamine and other allergy
mediators.
21. How might a drug advertised as a “histamine blocker” relieve allergy symptoms?
Histamine is an allergy mediator that causes blood vessels to dilate and triggers other
harmful effects of an allergic reaction. Histamine blockers would stop this chain
reaction.
22. Explain the difference between: clonal deletion and clonal selection; a natural killer
cell and a cytotoxic T cell; antibodies and antigens; cell-mediated and humoral immunity;
an autoimmune disorder and an immunodeficiency.
Clonal deletion occurs during lymphocyte development, when lymphocytes that respond
to antigens on the surface of the body’s cells are deleted; in clonal selection, a specific
antigen stimulates a B cell to divide. Natural killer cells are part of innate immunity;
cytotoxic T cells are part of adaptive immunity. Antibodies are Y-shaped proteins that
recognize antigens, which are molecules that trigger the production of antigens. In cellmediated immunity, pathogens are destroyed in direct cell-to-cell contact; in humoral
immunity, the main defense is antibodies. In an autoimmune disorder, the immune
system attacks the body’s own cells; in an immunodeficiency, the immune system is
lacking one or more essential components.
23. Search the Internet for evidence for and against the hypothesis that autism is
associated with the thimerosal preservative that was once added to childhood vaccines
(keep in mind that not all websites are equally credible). Do you find the evidence for or
against the hypothesis most compelling? What sort of evidence would it take to change
your position to the opposite side of the issue?
[Answers will vary]
24. Search the Internet for information about immune system disorders. Choose one
illness to study in more detail. What are the characteristics of the disorder? Who is
primarily affected? What causes the illness, and is there a treatment or cure?
[Answers will vary]
25. Search the Internet for ads for commercial products that claim to “boost the immune
system.” Choose a product to investigate in detail. What specific claims do ads for the
product make? What scientific evidence does the manufacturer offer in support of its
claims? Based on what you know about the immune system, does the scientific evidence
seem convincing?
[Answers will vary]
Pull it Together
1. How do innate defenses and adaptive immunity differ?
Innate defenses protect against all pathogens and are in place throughout life; they do not
change in their ability to protect you. Adaptive immunity is directed against specific
pathogens. The adaptive immune system changes throughout life by “remembering”
pathogens the body has already encountered.
2. What are examples of barriers that contribute to innate defenses?
Any barricade that can slow or stop the movement of an invader qualifies as innate
defense. Examples are intact skin, earwax, nose hairs, tears, mucus, and stomach acid.
3. How do cytokines interact with the adaptive immune system?
In general, cytokines are communication molecules that aid in the coordination of the
immune response. Interleukins are one type of cytokine. Macrophages release
interleukins that activate B and T cells.
4. How do lymph and lymph nodes fit into this concept map?
After a macrophage consumes a pathogen, it travels in lymph to a lymph node, where the
helper T cells trigger the adaptive immune response.
5. What are the roles of memory B cells and plasma B cells?
When they are first produced, memory B cells do not respond to the invading antigen.
Instead, they linger in the body, ready to launch a rapid response in case the same antigen
is encountered again. Plasma B cells are active as soon as they form, releasing specific
antibodies that neutralize an invading pathogen.